The counterfeiting of valuable documents such as bank notes, bonds, stock certificates, credit cards, and stamps has been, and continues to be, of major concern to government agencies, banking institutions and security firms.
This concern has greatly increased in the recent past as modern photocopy methods continue to improve. Present day photocopiers already have the capability to make faithful color reproductions of valuable documents which are of sufficient quality to pass a casual inspection by a lay observer. As a specific example dollar bills have been copied and the copies successfully used in dollar bill changers.
Continuing improvement in photocopy technology will exacerbate the problem of counterfeiting. It is anticipated that photocopies will be of such quality within the next five years that skilled personnel using the most sophisticated and advanced detection equipment will be required to detect counterfeit documents. As counterfeit documents are normally passed through lay personnel, lacking even rudimentary detection equipment, it is clear that counterfeiting through the use of photocopy techniques will soon become an uncontrollable problem.
Various techniques have been proposed in the past to permit detection by a lay person of a counterfeit document. Such techniques are described for example in U.S. Pat. No. 4,033,059 granted to R. G. Hutton et al. on July 5, 1977 and U.S. Pat. No. 4,124,947 granted to Adolf Kuhl et al. on Nov. 14, 1978. The technique described in both patents is similar and involves imprinting inked intaglio pattern elements, such as lines, on a document of value. As is well known, an intaglio imprint is obtained by pressing paper, or other substrates, against an engraved plate which is inked whereby the substrate is deformed (i.e. raised images are created) while being inked. The pattern elements are arranged to form background portions and readily recognizable image portions by making the pattern elements, which form the image portions, different in depth and orientation from the pattern elements which form the background portions. Varying orientation and depth of the pattern elements, in combination with inking, results in an image portion which blends with the background portion from one angle of view but which appears as a readily recognizable symbol from another angle of view. Such an image which varies in appearance when the angle of view is changed, is known as a "latent image."
A latent image, when imprinted with intaglio techniques on a document of value, is readily observed by a lay observer as the image appears and disappears as the observer varies his angle of view. This phenomenon results from the unique arrangement of pattern elements discussed above and is not observable on the portions of a document lacking the arrangement of pattern elements. However, when a photocopy is made of such a document it is, of course, made from a fixed angle of view. Therefore, the photocopy will contain only one aspect of the image, either the image of the readily recognizable symbol or the image blended into the background, with the copied aspect being dependent on which angle of view is chosen. Accordingly, a lay observer has no difficulty in detecting a photocopy counterfeit as the image will not change as the angle of view is varied.
Although the technique described in the U.S. patents discussed above has certain advantages over other methods of detecting counterfeit documents, several problems remain. First, the image must be ilnked to make it visible, with the color of the raised portion of the image necessarily being different than the color of the underlying document substrate. The necessity for inking adds expense and complexity to the process making it less desirable to a wide variety of users.
A second problem is that great care must be taken to create an image that is clearly visible from one angle of view and essentially invisible from a second angle of view. Unless great care is taken in creating the image it will be visible from any angle of view, completely destroying the advantage of detecting counterfeit documents with a latent image.
Still another problem with the prior art methods is if a color photocopy is made of the latent image the color of the copy may be the same as the original, although of course the latent image effect will be last. The fact that the colors are the same makes detection of a counterfeit document more difficult.
It is therefore an object of the present invention to provide a process for protecting valuable documents that does not have the disadvantages of the processes described above.
It is a further object of the present invention to provide a method for protecting valuable documents that is inexpensive and readily used by printers equipped for security printing.
It is a still further and general object of the present invention to provide a method for protecting valuable documents that can be used by a lay observer without the need for detection equipment.